1. Field of the Invention
The present invention relates to masks used in lithographic processing of semiconductor devices and, in particular, to a phase shifting mask which may be used to shift the phase of the energy beam employed in the lithographic process.
2. Description of Related Art
Photolithographic process used for producing semiconductor devices and the like utilize masks which are formed by depositing materials opaque to the energy beam to be utilized in a desired pattern on a substrate which is generally transparent to such energy beam. The mask pattern is transferred via a spatially modulated energy beam, e.g. light, to create an aerial image which is transferred to a resist film on a substrate. After being exposed to the pattern, the resist film is contacted with the developer and subsequently an etchant to create a pattern for the desired structure to be created on the substrate.
Such lithographic techniques are used to form fine (i.e. narrow) lines and features on the semiconductor wafer. In order to produce narrower lines, higher resolution methods must be utilized. One such method is to employ phase shifting masks to apply alternating 180xc2x0 phase differences to the energy being transmitted. When used on alternate transparent areas of the mask, such phase shifting masks reduce the minimum pattern resolution substantially, as compared to conventional photomask methods.
Alternating phase shifting masks are typically made by creating an alternating thickness of the transparent quartz substrate, normally by etching. Etching of the transparent quartz substrate provides difficulties in processing, particularly in controlling the depth of the etch and in repairing the quartz substrate. These difficulties are due in part to the fact that the quartz material of the substrate is only partially etched.
Some prior art phase shifting masks have employed spin-on-glass material added on to a patterned substrate. Such spin-on-glass material has not been particularly advantageous because it is difficult to maintain a uniform thickness when spin-coating a patterned substrate.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an improved alternating phase shifting mask for use in lithographic processing of semiconductor substrates.
It is another object of the present invention to provide a phase shifting mask in which there is improved control in manufacturing the alternating phase shifting regions.
It is a further object of the present invention to provide an improved method for making an alternating phase shifting mask.
The above and other objects and advantages, which will be apparent to one of skill in the art, are achieved in the present invention which is directed to, in a first aspect, a phase shifting mask for use in lithographic processing of semiconductor substrates comprising a mask substrate substantially transparent to the energy beam used in the lithographic processing and a patterned phase shifting layer disposed on the mask substrate and having openings therein exposing the mask substrate. The patterned phase shifting layer is comprised of a material of differing composition than the mask substrate and of thickness sufficient to shift the phase of an energy beam passing through the thickness of the patterned layer and the mask substrate by 180 degrees, compared to the phase of the energy beam passing through the phase shifting layer openings and the mask substrate. The mask also includes a patterned layer of a material substantially opaque to the energy beam disposed on the mask substrate or the patterned phase shifting mask layer.
The patterned phase shifting layer material is substantially transparent to the energy beam, and preferably has a transmissivity of at least about 98%, more preferably about 98% to about 99.5%, of the energy beam passing therethrough.
In its preferred embodiment, the mask substrate comprises quartz and the patterned phase shifting layer material comprises a silicon oxy nitride. In such case the patterned phase shifting layer thickness is about 100-200 nm.
In another aspect, the present invention relates to a phase shifting mask for use in lithographic processing of semiconductor substrates comprising a quartz mask substrate and a patterned phase shifting layer disposed on the quartz mask substrate and having openings therein exposing the quartz mask substrate. The patterned phase shifting layer is comprised of a silicon oxy nitride of thickness sufficient to shift the phase of an energy beam passing through the thickness of the patterned layer and the quartz mask substrate by 180 degrees, compared to the phase of the energy beam passing through the phase shifting layer openings and the quartz mask substrate, the silicon oxy nitride having a transmissivity of at least about 98% of the energy beam passing therethrough. The mask also includes a patterned layer of a material substantially opaque to the energy beam disposed on the quartz mask substrate or the patterned phase shifting mask layer. Preferably, the patterned phase shifting layer thickness is about 100-200 nm.
A related aspect of the present invention provides a method of making a phase shifting mask for use in lithographic processing of semiconductor substrates comprising providing a mask substrate and depositing on the mask substrate a patterned phase shifting layer. The patterned phase shifting layer has openings therein exposing the mask substrate. The patterned phase shifting layer is comprised of a material of differing composition than the mask substrate and of thickness sufficient to shift the phase of an energy beam passing through the thickness of the patterned layer and the mask substrate by 180 degrees, compared to the phase of the energy beam passing through the phase shifting layer openings and the mask substrate. The method also includes depositing on the mask substrate a patterned layer of a material substantially opaque to the energy beam disposed on the mask substrate or the patterned phase shifting mask layer.
The patterned phase shifting layer material is substantially transparent to the energy beam, and preferably has a transmissivity of at least about 98% of the energy beam passing therethrough. More preferably, the mask substrate comprises quartz and the patterned phase shifting layer material comprises a silicon oxy nitride, wherein the patterned phase shifting layer is deposited on the quartz mask substrate at a thickness of about 100-200 nm.
In one method of the present invention, the patterned phase shifting layer is deposited on the quartz mask substrate by depositing a uniform layer of the phase shifting material and etching selected areas of the uniform layer to form the patterned phase shifting layer, and during the etching the quartz mask substrate acts as an etch stop. Preferably, the etching of the phase shifting material is performed with a CF4+O2 or CH3F+O2 reactive ion etch (RIE) process.
A further aspect of the present invention provides a method of using a phase shifting mask for use in lithographic processing of semiconductor substrates comprising first providing a mask comprising a quartz mask substrate having a patterned phase shifting layer disposed thereon and having openings therein exposing the quartz mask substrate. The patterned phase shifting layer is comprised of a material of differing composition than the quartz mask substrate and of thickness sufficient to shift the phase of an energy beam passing through the thickness of the patterned layer and the quartz mask substrate by 180 degrees, compared to the phase of the energy beam passing through the phase shifting layer openings and the quartz mask substrate. There is also provided a patterned layer of a material substantially opaque to the energy beam disposed on the quartz mask substrate or the patterned phase shifting mask layer. The method then includes passing an energy beam through the quartz mask substrate and the patterned phase shifting layer and shifting the phase of the energy beam passing through the patterned phase shifting layer by 180xc2x0 compared to the phase of the energy beam passing through only the quartz mask substrate.
In this method the mask substrate preferably comprises quartz and the patterned phase shifting layer material preferably comprises a silicon oxy nitride, with the latter more preferably comprising SiO0-0.3N0.7-1.4.